A junction field effect transistor (JFET, also known as junction gate field effect transistor) is a semiconductor transistor formed in a semiconductor substrate and the current that flows in a JFET is controlled by electrodes made of P-N junctions formed in the semiconductor substrate. The gate electrode is one such P-N junction formed in the semiconductor substrate. Gate voltage is applied to a transistor channel across the P-N junction of the gate electrode and this controls the current going from source to drain. To turn off the transistor by pinching off the current flow, the gate-to-source voltage is controlled. To pinch off current in an N-channel JFET, a negative gate-to-source voltage is applied and to pinch off current flow in a P-channel JFET, a positive gate-to-source voltage is applied.
Junction field effect transistors have relatively long channels compared to MOSFET (Metal Oxide Semiconductor Field Effect Transistor) devices. In JFET transistors, the source and drain regions are not immediately adjacent the gate region or separated from the gate region by an LDD (lightly doped drain) region. Rather, the gate region in the substrate is separated from the source and drain regions also formed in the substrate. The transistor channel is a current path from the source to the drain as activated by the gate and therefore it is especially critical to position the source, drain and gate regions accurately so that they are accurately spaced apart by the desired spacing and a desired channel length is achieved.
The source and drain and gate regions are typically formed by separate patterning and ion implantation operations that introduce dopant impurities into the substrate and the separate patterning operations each carry with them a degree of alignment and placement inaccuracy. Any inaccuracy impacts the channel length which is designed in conjunction with the voltage to be applied to the gate. Therefore, if the gate is not spaced accurately from the source and the drain resulting in an undesired channel length, the transistor will not function properly using the desired voltage.
It is therefore desirable to produce transistors such as JFET's that have accurately aligned and carefully spaced features.